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How to design agitators for desired process response

| By Richard Hicks, Jerry Morton and John Fenic, Chemineer, Inc.

Effective agitation of fluids for blending and motion requires a detailed analysis of capacity, viscosity and dynamic response for the fluid system, in order to find power and shaft speed of the agitator and corresponding sized of the turbine impeller.

Our purpose in this article will be to emphasize a design procedure for practical problems in the chemical process industries involving blending and motion.

The information presented here may be used for the design of turbine agitators in applications ranging from storage vessels requiring very little agitation to critical reactors needing a great deal of it.

To understand the need for an organized approach to design, we will follow steps shown in the logic flow diagram of Figure 1 in deciding on an agitator for blending-and motion-problems. This illustration represents a portion of the overall logic flow diagram originally presented in part one of this series. We will review the mechanical design of drives, shafts and seals, and economic evaluations, in future articles of the series. Here, we will limit our discussion to procedures for blending and motion up to and including design of the impeller system.

Classification of problem

The design procedure for blending and motion applies to agitation problems where fluids behave as a single phase and where a predictable level of fluid motion must occur. For example, a typical blending problem may require the mixing to uniformity of fluids having dissimilar viscosity, density or concentration. On the other hand, a fluid motion problem may require improved convective heat transfer coefficients to facilitate heat removal from a reacting fluid.

The agitator design logic for blending and motion also applies to some two-phase systems that exhibit single-face behavior. An example would be fluids containing a very small concentration of solids and having very slow settling velocities.

It is equally important to state where the blending-and-motion logic is not applicable. The procedures in this article do not apply to problems such as immiscible liquid-liquid dispersion, or the blending of highly non-Newtonian fluids.

Following the design logic of the Figure 1, we will analyze each of the three components under the heading “magnitude of the agitation problem.”

For the rest of this article, which was first published by Chemical Engineering  in 1976, please fill out the form below.